1. Field of the Invention
The invention herein relates to the formation of fibrous calcium sulfate hemihydrate.
Research has been carried on for years on the conversion of gypsum (CaSO.sub.4.sup.. 2H.sub.2 O) into the various forms of its hemihydrate (CaSO.sub.4.sup.. 1/2H.sub.2 O), sometimes referred to as plaster of Paris, and its anhydrite (CaSO.sub.4). Because of the commercial importance of plaster of Paris as a construction material, most of the emphasis in this research has been upon processes to form hemihydrates having short, broad crystalline structures, for these tend to produce the strongest and most easily worked plasters. In recent years, however, emphasis has shifted from the production of plaster to the use of hemihydrate as reinforcement in other materials, notably organic polymeric matrices. For such usage, short, broad crystals of hemihydrate are unsatisfactory, despite their ability to form high strength plasters. As reinforcements, the fibrous forms of the hemihydrate have been found to be much superior in direct contrast to the respective properties of the two forms in plaster technology. For this reason, it would be desirable to develop processes which would produce high yields of fibrous hemihydrate while minimizing production of nonfibrous hemihydrate and anhydrite from gypsum.
2. Description of Prior Art
There is a wide variety of patents and articles discussing various aspects of hemihydrate formation from gypsum. Typical descriptive articles which describe the hemihydrates and their properties are those by Riddell in the May 1950 issue of Rock Products, page 68, and by Bauer in the April 1952 issue of Pit and Quarry, page 113. The book The Chemistry of Cement and Concrete by Lea describes properties of gypsum and its derivatives on page 18. Various aspects of the formation of hemihydrate by calcination of gypsum at high temperature in the presence of steam under pressure are disclosed in U.S. Pats. Nos. 1,901,051; 1,931,240; 1,941,188; 1,989,712; 2,448,218; 2,460,267; 3,423,172; 3,576,599; and 3,579,300. Hemihydrate crystalline forms are described in U.S. Pats. Nos. 2,616,789 and 3,580,703. Formation of fibrous hemihydrate from high concentration suspensions of gypsum is described in an article by Eberl et al in the May 1949 issue of Industrial and Engineering Chemistry, page 1061. Crystal habit modifiers are described in U.S. Pats. Nos. 2,907,668 and 3,520,708.